Hi Patrick
the loudness sensor can not turn the sound into db. it is only outputs to voltage. we can not map the voltage to db accurately. you can refer to http://wiki.seeedstudio.com/Grove-Loudness_Sensor/ for more detail info.
for the respeaker pi hat, we do not use it for the db meter before. i see that there is sound meter python library. https://pypi.org/project/soundmeter/. please check if it works. thanks.